What is the point of the first scene literally and in reference to the whole play?

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Just as the Porter in Act 2 extemporizes about the sin of equivocation, the play figures equivocation as one of its most important themes. Starting from the Weird Sisters' first words that open the play, audiences quickly ascertain that things are not what they seem. This sets up one of the major themes of the play. This also sets up the fact that Macbeth will be "played" with by the witches using riddles and equivocation.